BUG ? git-log omits changesets if the file is identical between revs

[Darren Hart <dvhltc@us.ibm.com>]

Looking at the linux-tip git tree, I was having trouble understanding
the following results while trying to track down a kernel bug:

URL: git://git.kernel.org/pub/scm/linux/kernel/git/x86/linux-2.6-tip.git

$ git checkout -b 2.6.33.5-rt23 v2.6.33.5-rt23
$ git log --pretty=oneline v2.6.33.5.. arch/powerpc/kernel
ef4afcc4401e92982f4ffd3fae6a9a6745a98596 Merge stable/linux-2.6.33.y into rt/2.6.33
1540c84b5ed657ed71dce06915bba461e6b09574 Merge branch '2.6.33.4' into rt/2.6.33
ac4d7f0bed9b03204539de81c8b62b35c9f4e16a Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git
6aa4659865454a3885195aa49348c46a69e6bee2 powerpc: rtasd: do not check for irq_disabled on RT
5f854cfc024622e4aae14d7cf422f6ff86278688 Forward to 2.6.33-rc8
5a017e25b44f6e66b0254c84270c0f1bf8689ebe Merge commit 'v2.6.31' into rt/head
570f410b475b45462197d09ddca25e47616da5ac Merge branch 'rt/base' into rt/rt-2.6.31
bda9263432072d7879fe13fdaf9dbbd372b4d2fc Merge branch 'linus' into rt/base
92bfe2d63ed46764b0b0a6e9b63bb9e81023ab80 powerpc: realtime support
e7e7fcb92fc6ef2cdb8cc070e9c0d26f03f7b8eb powerpc: mmu gather and tlb fixes
b4f4919b32aeadc6e0c1a0bc996e541073e0bcf7 powerpc: kprobes: Fix missed preemption check
5b659531c96943c3eea21b3dbcb83e6320d0f46b powerpc: OF convert to atomic lock
32ff9169c2f6c94c900f08217359972e2f3eb17f powerpc: Annotate atomic_locks
5a950072e4c1036abcdb35610d053e49bdde55c9 xtime_lock: Convert to atomic_seqlock
fd2bde5dd1689cc8ede833604cc19d1c835faf61 genirq: Convert irqdesc lock to atomic_spinlock

This lists the following change of interest:
92bfe2d63ed46764b0b0a6e9b63bb9e81023ab80 powerpc: realtime support

$ git whatchanged 92bfe2d6 -n1
commit 92bfe2d63ed46764b0b0a6e9b63bb9e81023ab80
Author: Thomas Gleixner <tglx@linutronix.de>
Date:   Fri Jul 3 08:44:51 2009 -0500

    powerpc: realtime support
    
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

:100644 100644 d00131c... 0b46b68... M  arch/powerpc/Kconfig
:100644 100644 43e0734... 4bb9ce4... M  arch/powerpc/kernel/entry_64.S
:100644 100644 88d9c1d... 1a82d48... M  arch/powerpc/kernel/idle.c
:100644 100644 054dfe5... 8f1d8cd... M  arch/powerpc/platforms/chrp/time.c
:100644 100644 e1f33a8... db7cf0d... M  arch/powerpc/xmon/xmon.c

Note that this change modified arch/powerpc/kernel/entry_64.S. However,
if we log the changes to that single file with same revision range as
for the parent directory above, we get an empty result set:

$ git log --pretty=oneline v2.6.33.5.. arch/powerpc/kernel/entry_64.S
<null>

Is this perhaps because entry_64.S is now identical between v2.6.33.5
and v.2.6.33.5-rt23?

$ git diff v2.6.33.5.. arch/powerpc/kernel/entry_64.S
<null>

If so, is this perhaps a bug? It seems like 92bfe2d6 should show up in
the log regardless...

I'm not sure of the significance, but another point of reference, using
the ... rev syntax yields:
$ git log --pretty=oneline v2.6.33.5... arch/powerpc/kernel/entry_64.S
92bfe2d63ed46764b0b0a6e9b63bb9e81023ab80 powerpc: realtime support

If this is all expected behavior, could someone please explain why it is
so?

Regards,

-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team